In recent years in the field of automotive internal combustion engines, it has been desirable to improve exhaust purification. In particular, improvements for cold-start exhaust purification technologies suited to a time period from a cold engine start during which a catalyst is in an inactive state have been sought. Further, as stringent legal regulations regarding exhaust emissions have been implemented, the need for improved exhaust purification has become more pronounced. Therefore, when starting with a cold engine, to rapidly activate a catalyst, a catalyst temperature increase control, such as, for example, an idle speed increase control or an ignition timing retard control is performed. However, there is a need to perform a diagnosis operation to ensure that such control functions are performing normally.
An example of a diagnostic technology has been disclosed in Japanese Patent Provisional Publication No. 2001-132526. In this reference, during a cold engine start, engine speed and ignition timing are both monitored. A determination of a failure (a malfunction) is made when the monitored engine speed, after an elapse of a predetermined time period from the start of the operation of a catalyst temperature increase, is less than or equal to a predetermined value or when the monitored ignition timing (spark-advance value) is greater than or equal to a predetermined value.
However, in the system disclosed in the above-mentioned reference, the diagnostic operation cannot be initiated unless the predetermined delay time has expired under a specific engine operating condition, such as an idling operation, where engine speed feedback control and ignition timing feedback control are both executed. For example, when shifting to an acceleration-running mode for a comparatively short time period after the cold engine start, the diagnosis is not performed. Thus, there is a need for further improvement.